The 10 mm 10 vol.-% SiCp/AZ91 composite was fabricated by stir casting. After processing by forging and subsequent extrusion, the composite with 1,2 mm grain size was obtained. Then the hot deformation behaviour of as-extruded composite was investigated at elevated temperatures (543-673 K). Results show that the existence of micrometre SiCp has obvious effect on stimulating dynamic recrystallization (DRX) nucleation and inhibiting the growth of DRXed grains. Both the flow stress and work hardening of as-extruded composite is lower than that of as-cast AZ91 alloy and composite, which means that the grain refinement strengthening mechanism is not fit for elevated temperatures, and the reasons were analysed. The calculated activation energy Q (,90 KJ mol 21 ) of as-extruded composite is very close to the grain boundary diffusion activation energy (,92 KJ mol 21 ) for Mg. Calculation analysis gives n55, which illustrates that the climb of dislocation is the main controlling mechanism of the as-extruded composite during hot deformation.